German Published Patent Application No. 198 38 886 describes an electric immobilizing brake or parking brake for a vehicle. The driver of the motor vehicle activates the electric parking brake via a push-button switch, for example. The parking brake provides that the vehicle is no longer able to move in the stopped or parked state. To achieve the immobilizing braking effect, it is possible, for example, to control the parking brake's wheel brakes arranged on the wheels of the motor vehicle by electromotor/transmission units designed to self-lock. This provides the effect of a conventional mechanical immobilization brake.
Activation of the electric parking brake requires electric energy, which can be drawn from the vehicle electrical system. When turning off or parking the motor vehicle, in particular on inclines, it is important that the electrical energy is available at all times. Otherwise it may happen that the vehicle is not reliably protected from unintended rolling. Dangerous situations may occur as a result, in particular in motor vehicles in which the mechanical power of the drive motor is additionally utilized to activate a service brake of the motor vehicle. If after shut-down of the drive motor the electrical energy in such vehicles is no longer sufficient to restart the drive motor, the vehicle is no longer fully controllable.
To avoid such situations, German Published Patent Application No. 198 38 886 describes to provide an auxiliary battery whose energy, as a minimum, is sufficient to activate the parking brake. Disadvantageous in an auxiliary battery provided in addition to the battery of the vehicle electrical system is the effort and expense it entails with regard to construction and production. Also, it is possible once again that the auxiliary battery will no longer have sufficient energy or is unable to store sufficient energy.